Nature is replete with illustrations of the power of bio/organic interfaces to mediate nanostructure formation in organic, inorganic, and composite materials. Some common examples include bones, mollusk shell, teeth and avian eggshell. Such natural wonders have spurred leading scientists to pursue similar biomimetic routes for synthesis of advanced materials. Self-assembled molecular monolayers (SAMs) are beginning to be used as artificial versions of the “magical” natural interfaces, and are being investigated for synthesis of thin films for optical or sensing applications, nanocomposites for structural/thermal applications, and inorganic membranes for separations and catalysis.
Oxide ceramic layers or films on solid substrates have found significant applications in corrosion protection, chemical sensors, insulating films for capacitors, inorganic membranes for gas separations and catalysis, and in micro and optoelectronic devices. The useful applications of thin films are a result of specific material properties such as chemical and thermal stability, high refractive index, high damage threshold, or a high melting point. There is a great demand for oxide thin films that are structurally uniform, dense and adherent to solid substrates. The methods of oxide thin film fabrication include a variety of techniques, such as reactive e-beam processes, galvanostatic oxidation, dual ion beam sputtering, and atomic layer epitaxy. These methods typically require conditions of high temperature and vacuum. An alternative route for the synthesis of oxide thin films involves the use of aqueous chemical solutions and self-assembled monolayers (SAMs), a biomimetic or bio-inspired process.
The biomimetic processing is an area of increased interest in materials research due to the demand for new ceramic materials and more effective techniques for their production. This approach is inspired by immobilization that in general involves the formation of well-structured and complex-shaped organic/inorganic composites by the deposition of an inorganic solid on an organic matrix that consists of biomolecules like proteins. Biomineralization (i.e., the exquisite control over inorganic solid nucleation, crystallization and growth at organic interfaces in aqueous environments) usually occurs at ambient conditions with respect to temperature, pressure, and atmosphere. As a research tool, SAMs of organic molecules are utilized to mimic the contribution of organic surfaces/interfaces in the natural biomineralization processes. The chemical solution deposition of inorganic materials onto SAMs and amphiphilic structures has been suggested by others. The use of solution deposition on a solid substrate has several advantages over other film deposition techniques, including ambient conditions, possibility of one-step soft solution processing, more cost effectiveness, tailored chemical constituency, easiness to introduce impurities for phase stabilization, uniform film deposition on objects of complicated geometry or temperature-sensitive substrates, potential for patterned growth of films on solid substrates, and possible spontaneous formation of a highly complex, uniform large area structure.
Organic SAMs, such as those described by Agarwal et al., incorporated herein by reference, are highly ordered two-dimensional arrays of long-chain hydrocarbon molecules (X—(CH2)n—Y) of a specific length, which are covalently attached to a substrate through X-end functional group and possess a functional surface group Y that is projecting away from the substrate surface. The highly ordered and close-packed characteristics of the monolayer are a result of the strong interactions that exist between the substrate and the monolayer in addition to the short-range van der Waals forces between the chains. The functional terminal group Y on the SAM surface can be chemically modified without disturbing the monolayer to provide a favorable surface functionality necessary to initiate and promote deposition of metal oxide film from the surrounding solutions/colloidal suspensions. In addition to the ability to provide a desired surface functionality, SAMs can withstand temperatures up to 100° C. and solutions that are strongly acidic or basic. Readily prepared on a large scale, SAMs are mechanically and solvolytically stable. With these characteristics, SAMs can be used in a variety of conditions for the deposition of various oxides from aqueous or some non-aqueous solutions. SAMs can be patterned to selectively deposit inorganic films on desirable regions of substrate surface via controlled location of nucleation and orientation. In addition, SAMs can form on nanoscale curved surfaces and thus mediate the uniform growth or deposition of an inorganic film on the surface of a nanoparticle
One of the fundamental, but critical problems is to how to lay down the high-quality nanostructured inorganic film on the organic surfaces of SAMs that already forms on a substrate. By definition, nanostructured films can be thin films with thickness between 1 to 100 nanometers, nanocrystalline films, films that contain nanoclusters, nanoporous films, or patterned films with nanoscale feature sizes. Nanostructured films clearly have shown great impact upon many technologies and applications. DeGuire et al. (1994, 1996, 1998) have achieved success in growing zirconia, titania, zinc oxide, tin oxide, and iron oxide thin films on substrate (silicon) surfaces via SAM interfaces (Agarwal et. al., 1997; Shin et al., 1998). Several other research groups also showed that through SAM interfaces, titania film can be grown on substrates like glasses and polymers. No one has ever attempted or mentioned the possibility of hafnia deposition by the SAM-mediated approach.
Previous research has demonstrated that thin films of various oxides (such as ZrO2, Y2O3-Doped ZrO2, ZnO, SnO2, TiO2, FeOOH, and SiOx) and non-oxides (such as sulfides, silicon-dicarbodiimide, and GaN) can be deposited on self-assembled monolayers on solid substrates submerged in solutions. For example, wafers grafted with SAMs terminated with hydrophilic sulfonate groups (SO3−) were immersed into acidic aqueous solutions of zirconium sulfate, and a film layer of zirconia nanocrystals with amorphous admixtures of basic zirconium sulfate was deposited within several hours. The zirconia films were dense, adherent and could be converted into purely tetragonal phase by calcinations at 773K for 2 hours.
Oxide form of hafnia is a versatile material that shares a lot of desired thermal, mechanical, and chemical stability properties similar to those for zirconia (ZrO2), such as high toughness, good refractory behavior, low thermal conductivity, and high oxygen ion conductivity at elevated temperatures. Corresponding to these unique properties, applications have been found in wear-resistant coatings, thermal barrier coatings, solid-oxide fuel cells, and oxygen sensors. In fact, HfO2 is expected to be thermally tougher because it has a higher transition temperature (monoclinic to tetragonal structure at 1700° C.) and a lower expansion coefficient than ZrO2. HfO2 has a superb thermodynamic and chemical stability and a high melting point. Furthermore, HfO2 has unique electrical properties as well as very high refractive index and excellent UV transparency (similar to titania) that could allow its use in optical coatings or electric/optical thin-film based devices, such as optical nanowaveguide/interconnects for future-generation nanoelectronic circuits.
There is a popular demand for a method that allows preparation of high-quality thin films, which are structurally uniform on a large scale, dense and adherent to the substrate. The methods that have been reported so far for hafnia thin film fabrication include: dual ion beam sputtering (Capone et al., 1998), electrochemical techniques (potentiodynamic, galvanostatic, and potentiometric conditions) (Esplandiu et al., 1997; 1995a; 1995b), reactive and conventional electron beam evaporation (Tsou and Ho, 1996; Chow et al., 1993), reactive sputtering (Platt et al., 1996), pulsed laser ablation (Reisse et al., 1996); reactive thermal evaporation (Tcheliebou et al., 1993), and atomic layer epitaxy (Kukli et al., 1996). Unfortunately, hafnia film fabrication technique via a cost-effective low-temperature route (i.e. SAM-mediated solution deposition) has not been yet been reported.